Sealants useful in aerospace and other applications must satisfy demanding mechanical, chemical, and environmental requirements. The sealants can be applied to a variety of surfaces including metal surfaces, primer coatings, intermediate coatings, finished coatings, and aged coatings. Sealant compositions comprising sulfur-containing polymers that exhibit acceptable fuel resistance, thermal resistance, and flexibility for aerospace sealant applications are described in U.S. Pat. No. 6,172,179. In sealants such as those described in U.S. Publication Nos. 2006/0270796, 2007/0287810, and 2009/0326167, a sulfur-containing polymer such as a thiol-terminated polythioether is reacted with an epoxy curing agent in the presence of an amine catalyst to provide a cured product. These systems are useful as sealants and can meet the demanding performance requirements of the aerospace industry. Without a strong base catalyst, such as an amine catalyst, the reaction between the thiol and epoxy groups is slow and provides a relatively long pot life, for example, of several days to weeks, depending on the temperature. However, the physical properties of the cured sealants are generally not acceptable. In contrast, in the presence of a strong base catalyst the reaction is fast, and although exhibiting acceptable cured properties, provides a pot life of only about 2 hours to about 12 hours depending upon the particular system. For many applications, however, a longer pot life such as from 12 hours to 48 hours is desirable.
In practice, the compositions can be provided as two-part compositions in which the thiol-terminated sulfur-containing compound and the epoxy are provided as separate components, with the amine catalyst in the thiol component, and the two parts are mixed shortly prior to use. Alternatively, the base catalyst may be provided as a third component, and the component containing the thiol-terminated sulfur-containing polymer, the component containing the epoxy, and the component containing the base catalyst are mixed shortly before use. However, once the components are mixed, the thiol and epoxy groups react, and depending at least in part on the temperature and on the type of amine catalyst, the pot life is limited to less than 2 to 12 hours. Furthermore, as the composition cures, there is little ability to control the reaction rate to take advantage of the complex chemistries taking place after the sealant is applied to a surface.
Ways to prolong the pot life and to control the curing rate of compositions comprising thiol-terminated sulfur-containing polymers and polyepoxides are desired.